lecture6-scheduling · scheduling cse 30341 – operang system principles 2 objecves • to...

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CSE30341Opera,ngSystemPrinciples

Scheduling

CSE30341–Opera,ngSystemPrinciples 2

Objec,ves

•  TointroduceCPUscheduling,whichisthebasisformul,-programmedandmul,-taskingsystems

•  TodescribevariousCPU-schedulingalgorithms

•  Todiscussevalua;oncriteriaforselec,ngaCPU-schedulingalgorithmforapar,cularsystem

•  Toexaminetheschedulingalgorithmsofseveralopera,ngsystems

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WhySchedule?

Firefox

Word

Thunderbird

Spo,fy

CPU1

CPU2

Scheduler

Whoshouldrun?

Whereshouldtheprocess(thread)run?

I/O

WhatdoIwanttoop,mize?

ResponsiveMostWorkDone

ScaleWell


Burs,ness

•  Computa,onandI/Otendtobebursty– Readsomedata– Computeabunch– Writesomedata– Repeat

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“KeepCPUbusy”

Process1

Process2

CPUProcess1 Process2


Scheduling

Scheduler

ReadyQueue

Jobsarereadyforcomputa,on“NeedCPU”

Shorttermscheduler

CPU

Runningprocess

WhendoIchoosethenextonetorun?

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SchedulingChoices

•  Non-preemp,ve(voluntarily):– Processyields– Processgoesfromrunningtowai,ngstate– Processterminates

•  Preemp,ve(forced,canhappenany,me):– OSforcesprocessfromrunningtoreadystate


Dispatcher

•  MechanismthatgivescontroloftheCPUtoselectedprocess;includes:– Contextswitch•  Save/restorestack,registers,…

– Switchbacktousermode– ResumePCforprocess

DispatchLatency:Timeittakestostoponeprocessandswaptoanother

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SchedulingCriteria•  CPUu;liza;on–keeptheCPUasbusyaspossible(“howbusyis

theCPU”)

•  Throughput–#ofprocessesthatcompletetheirexecu,onper,meunit(“howmuchworkisgefngdone”)

•  Turnaround;me–amountof,metoexecuteapar,cularprocess(“howlongdoesittaketoexecuteaprocess”)

•  Wai;ng;me–amountof,meaprocesshasbeenwai,ngintheREADYQUEUE(RUNQUEUE)

•  Response;me–amountof,meittakesfromwhenarequestwassubmijedun,lthefirstresponseisproduced


SchedulingAlgorithmOp,miza,onCriteria

•  MaxCPUu,liza,on•  Maxthroughput•  Minturnaround,me•  Minwai,ng,me•  Minresponse,me

Thefancierwegetop,miza,on-wise,themorecomplexitywehavein

thesystem

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First-Come,First-Served(FCFS)Scheduling

ProcessBurstTime P1 24 P2 3 P3 3•  SupposethattheprocessesarriveintheorderP1,P2,P3andreadyforexecu,onat,me0

Wai;ngTime

P1=0;P2=24;P3=27

AverageWai;ngTime(0+24+27)/3=17

Computa,onTime

P1 P2 P3

0 24 27 30


DifferentOrder

•  SupposethattheprocessesarriveintheorderP2,P3,P1(readyforexecu,onat,me0)

Wai;ngTimeP1=6;P2=0;P3=3


ConvoyEffect

P1P3P2

0 3 6 30

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FCFS(FIFO)

•  Verysimple(addprocessestoendofrunqueue,takeprocessesfrombeginningofqueue)

•  Note:processesreturningfromwaitqueuesalwaysgotothebackoftherunqueue!

•  NON-PREEMPTIVE(,me-sharing/interac,ve?)


Shortest-Job-First(SJF)Scheduling

•  AssociatewitheachprocessthelengthofitsnextCPUburst–  Usetheselengthstoscheduletheprocesswiththeshortest,me

•  SJFisop;mal–givesminimumaveragewai;ng;meforagivensetofprocesses– ThedifficultyisknowingthelengthofthenextCPUrequest

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Revisi,ngwithSJF

ProcessBurstTime P1 24 P2 3 P3 3•  SupposethattheprocessesarriveintheorderP1,P2,P3(allinqueueat,me0)



Computa,onTime

P1P3P2

0 3 6 30


Shortest-JobFirst

Process BurstTime P1 6P2 8P3 7P4 3

P3P1P4

0 3 9 24

P2

16


AverageWai;ngTime(3+16+9+0)/4=7

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DeterminingLengthofNextCPUBurst•  Canonlyes,matethelength–shouldbesimilartothepreviousone

–  ThenpickprocesswithshortestpredictednextCPUburst

•  CanbedonebyusingthelengthofpreviousCPUbursts,usingexponen,alaveraging

•  Commonly,α setto½

:Define 4.10 , 3.

burst CPU next the for value predicted 2.burst CPU of length actual 1.

≤≤

=

=

+

αα

τ 1n

thn nt

( ) .1 1 nnn t ταατ −+==

AlsocalledEWMA

Exponen,alWeightedMovingAverage


Exponen,alAveraging

•  α = 0? α = 1?

τn+1 = α*tn + (1-α) α*tn-1 + (1-α)2*αtn-2… + (1-α)j αtn-j + …… + (1-α)n+1τ0

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Predic,onoftheLengthoftheNextCPUBurst

6 4 6 4 13 13 13 …810 6 6 5 9 11 12 …

CPU burst (ti)

"guess" (τi)

ti

τi

2

time

4

6

8

10

12


ShortestJobFirst(SJF)•  Non-preemp;ve!•  Preemp,veversioncalledshortest-remaining-;me-firstProcess-Arrival/BurstTime P1 0/8P2 1/4P3 2/9P4 3/5

P4P2P1

0 10 26

P1

171 5

P3

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PriorityScheduling•  Aprioritynumber(integer)isassociatedwitheachprocess•  TheCPUisallocatedtotheprocesswiththehighestpriority

(smallestinteger≡highestpriority)–  Preemp,ve–  Non-preemp,ve

•  SJFispriorityschedulingwherepriorityistheinverseofpredictednextCPUburst,me

•  Fixedpriori;es:donotchangeover,me•  Dynamicpriori;es:canchangeover,me•  Problem:Starva;on–lowpriorityprocessesmayneverexecute•  Solu,on:Aging–as,meprogressesincreasethepriorityofthe

process


ExampleofPriorityScheduling

ProcessAarriBurstTimeTPriority P1 10 3 P2 1 1 P3 24 P4 15 P5 52•  Lownumber=highpriority•  Dynamicversussta,c/fixedpriority

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RoundRobin(RR)•  Switchbetweenprocessesata,meinterval–  Timequantum,q–  10-100ms–  Preemp;ve

•  Whatdoesitmean?– Ntasks?– Maximumwait,me

•  Performance–  qlarge⇒FIFO–  qsmall⇒qmustbelargewithrespecttocontextswitch,otherwiseoverheadistoohigh


ExampleofRRwithTimeQuantum=4 Process BurstTime P1 24 P2 3 P3 3

•  TheGanjchartis:

•  Typically,higheraverageturnaroundthanSJF,butbejerresponse•  qshouldbelargecomparedtocontextswitch,me•  qusually10msto100ms,contextswitch<10usec

P1 P2 P3 P1 P1 P1 P1 P10 4 7 10 14 18 22 26 30

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TimeQuantumandContextSwitchTime


TurnaroundTimeVariesWithTheTimeQuantum

80%ofCPUburstsshouldbeshorterthanq

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Mul,levelQueue•  Readyqueueispar,,onedintoseparatequeues,e.g.:

–  foreground(interac,ve)–  background(batch)

•  Processpermanentlyinagivenqueue•  Eachqueuehasitsownschedulingalgorithm:

–  foreground–RR–  background–FCFS

•  Schedulingmustbedonebetweenthequeues:–  Fixedpriorityscheduling;(i.e.,serveallfromforegroundthenfrom

background).Possibilityofstarva;on!–  Timeslice–eachqueuegetsacertainamountofCPU,mewhichit

canscheduleamongstitsprocesses;i.e.,80%toforegroundinRR–  20%tobackgroundinFCFS


Mul,levelQueueScheduling

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Mul,levelFeedbackQueue

•  Aprocesscanmovebetweenthevariousqueues;agingcanbeimplementedthisway

•  Mul,level-feedback-queueschedulerdefinedbythefollowingparameters:–  numberofqueues–  schedulingalgorithmsforeachqueue–  methodusedtodeterminewhentoupgradeaprocess–  methodusedtodeterminewhentodemoteaprocess–  methodusedtodeterminewhichqueueaprocesswillenterwhenthatprocessneedsservice


ExampleofMul,levelFeedbackQueue

•  Threequeues:–  Q0–RRwith,mequantum8milliseconds–  Q1–RR,mequantum16milliseconds–  Q2–FCFS

•  Scheduling–  AnewjobentersqueueQ0whichisservedFCFS

•  WhenitgainsCPU,jobreceives8milliseconds•  Ifitdoesnotfinishin8milliseconds,jobismovedtoqueueQ1

–  AtQ1jobisagainservedFCFSandreceives16addi,onalmilliseconds•  Ifits,lldoesnotcomplete,itispreemptedandmovedtoqueueQ2

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ThreadScheduling

•  Dis,nc,onbetweenuser-levelandkernel-levelthreads

•  Kernel-level:threadsscheduled,notprocesses–  system-conten;onscope(SCS)–compe,,onamongallthreadsinsystem

•  User-level:threadlibraryschedulesuser-levelthreadstorunon“LWP”(“lightweightprocess”)–  calledprocess-conten;onscope(PCS)sinceschedulingcompe,,oniswithintheprocess

–  typicallydoneviaprioritysetbyprogrammer


PthreadScheduling

•  APIallowsspecifyingeitherPCSorSCSduringthreadcrea,on– PTHREAD_SCOPE_PROCESSschedulesthreadsusingPCSscheduling

– PTHREAD_SCOPE_SYSTEMschedulesthreadsusingSCSscheduling

•  CanbelimitedbyOS–LinuxandMacOSXonlyallowPTHREAD_SCOPE_SYSTEM

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PthreadSchedulingAPI#include <pthread.h> #include <stdio.h> #define NUM_THREADS 5 int main(int argc, char *argv[]) { int i, scope; pthread_t tid[NUM_THREADS]; pthread_attr_t attr; /* get the default attributes */ pthread_attr_init(&attr); /* first inquire on the current scope */ if (pthread_attr_getscope(&attr, &scope) != 0) fprintf(stderr, "Unable to get scheduling scope\n"); else { if (scope == PTHREAD_SCOPE_PROCESS) printf("PTHREAD_SCOPE_PROCESS"); else if (scope == PTHREAD_SCOPE_SYSTEM) printf("PTHREAD_SCOPE_SYSTEM"); else fprintf(stderr, "Illegal scope value.\n"); }


PthreadSchedulingAPI /* set the scheduling algorithm to PCS or SCS */ pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM); /* create the threads */ for (i = 0; i < NUM_THREADS; i++) pthread_create(&tid[i],&attr,runner,NULL); /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) pthread_join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { /* do some work ... */ pthread_exit(0); }

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Mul,ple-ProcessorScheduling•  CPUschedulingmorecomplexwhenmul,pleCPUsareavailable•  Homogeneousprocessorswithinamul,processor•  Asymmetricmul;processing–onlyoneprocessoraccessesthe

systemdatastructures,allevia,ngtheneedfordatasharing•  Symmetricmul;processing(SMP)–eachprocessorisself-

scheduling,allprocessesincommonreadyqueue,oreachhasitsownprivatequeueofreadyprocesses–  Currently,mostcommon

•  Processoraffinity–processhasaffinityforprocessoronwhichitiscurrentlyrunning–  so[affinity–  hardaffinity–  “migra;on”:processchangesprocessor


NUMAandCPUScheduling

CPU

fast access

memory

CPU

fast accessslow access

memory

computer

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LoadBalancing

•  IfSMP,needtokeepallCPUsloadedforefficiency

•  Loadbalancingajemptstokeepworkloadevenlydistributed

•  Pushmigra;on–periodictaskchecksloadoneachprocessor,andifimbalancedfoundpushestaskfromoverloadedCPUtootherCPUs

•  Pullmigra;on–idleprocessorspullswai,ngtaskfrombusyprocessor


Mul,coreProcessors

•  Recenttrendtoplacemul,pleprocessorcoresonsamephysicalchip

•  Fasterandconsumeslesspower

•  Mul,plethreadspercorealsogrowing–  Takesadvantageofmemorystalltomakeprogressonanotherthreadwhilememoryretrievehappens

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Mul,threadedMul,coreSystem


Real-TimeScheduling

Deadline

Process,me

Deadline

Process,me

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Priority-BasedScheduling•  Manyreal-,meprocessesareperiodic,i.e.,theyrequireCPU

atconstantintervals–  Hasprocessing,met,deadlined,periodp–  0≤t≤d≤p–  Rateofperiodictaskis1/p


RateMontonicScheduling

•  Apriorityisassignedbasedontheinverseofitsperiod

•  Shorterperiods=higherpriority•  Longerperiods=lowerpriority

•  P1isassignedahigherprioritythanP2.

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MissedDeadlineswithRateMonotonicScheduling


EarliestDeadlineFirstScheduling(EDF)

•  Priori,esareassignedaccordingtodeadlines:–  theearlierthedeadline,thehigherthepriority–  thelaterthedeadline,thelowerthepriority

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POSIXReal-TimeScheduling■  ThePOSIX.1bstandard

■  APIprovidesfunc,onsformanagingreal-,methreads

■  Definestwoschedulingclassesforreal-,methreads:

1.  SCHED_FIFO-threadsarescheduledusingaFCFSstrategywithaFIFOqueue.Thereisno,me-slicingforthreadsofequalpriority

2.  SCHED_RR-similartoSCHED_FIFOexcept,me-slicingoccursforthreadsofequalpriority

■  Definestwofunc,onsforgefngandsefngschedulingpolicy:

1.  pthread_attr_getsched_policy(pthread_attr_t *attr, int *policy)

2.  pthread_attr_setsched_policy(pthread_attr_t *attr, int policy)


POSIXReal-TimeSchedulingAPI#include <pthread.h> #include <stdio.h> #define NUM_THREADS 5 int main(int argc, char *argv[]) { int i, policy; pthread_t tid[NUM THREADS]; pthread_attr_t attr; /* get the default attributes */ pthread_attr_init(&attr); /* get the current scheduling policy */ if (pthread_attr_getschedpolicy(&attr, &policy) != 0) fprintf(stderr, "Unable to get policy.\n"); else { if (policy == SCHED_OTHER) printf("SCHED_OTHER\n"); else if (policy == SCHED_RR) printf("SCHED_RR\n"); else if (policy == SCHED_FIFO) printf("SCHED_FIFO\n"); }

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POSIXReal-TimeSchedulingAPI(Cont.)

/* set the scheduling policy - FIFO, RR, or OTHER */ if (pthread_attr_setschedpolicy(&attr, SCHED_FIFO) != 0) fprintf(stderr, "Unable to set policy.\n"); /* create the threads */ for (i = 0; i < NUM_THREADS; i++) pthread_create(&tid[i],&attr,runner,NULL); /* now join on each thread */ for (i = 0; i < NUM_THREADS; i++) pthread_join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { /* do some work ... */ pthread_exit(0); }


LinuxSchedulingThroughVersion2.5•  Priortokernelversion2.5,ranvaria,onofstandardUNIX

schedulingalgorithm•  Version2.5movedtoconstantorderO(1)scheduling,me

–  Preemp,ve,prioritybased–  Twopriorityranges:,me-sharingandreal-,me–  Real-;merangefrom0to99andnicevaluefrom100to140–  Mapintoglobalprioritywithnumericallylowervaluesindica,ng

higherpriority–  Higherprioritygetslargerq–  Taskrun-ableaslongas,mele{in,meslice(ac;ve)–  Ifno,mele{(expired),notrun-ableun,lallothertasksusetheir

slices–  Allrun-abletaskstrackedinper-CPUrunqueuedatastructure

•  Twopriorityarrays(ac,ve,expired)•  Tasksindexedbypriority•  Whennomoreac,ve,arraysareexchanged

–  Workedwell,butpoorresponse,mesforinterac,veprocesses

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LinuxSchedulinginVersion2.6.23+

CompletelyFairScheduler(CFS)•  Schedulingclasses

–  Eachhasspecificpriority–  Schedulerpickshighestprioritytaskinhighestschedulingclass–  Ratherthanquantumbasedonfixed,meallotments,basedonpropor,onofCPU,me–  2schedulingclassesincluded,otherscanbeadded

1.  default2.  real-,me

•  Quantumcalculatedbasedonnicevaluefrom-20to+19–  Lowervalueishigherpriority–  Calculatestargetlatency–intervalof,meduringwhichtaskshouldrunatleastonce–  Targetlatencycanincreaseifsaynumberofac,vetasksincreases

•  CFSschedulermaintainspertaskvirtualrun;meinvariablevruntime –  Associatedwithdecayfactorbasedonpriorityoftask–lowerpriorityishigherdecayrate–  Normaldefaultpriorityyieldsvirtualrun,me=actualrun,me

•  Todecidenexttasktorun,schedulerpickstaskwithlowestvirtualrun,me


lecture6-scheduling · scheduling cse 30341 – operang system principles 2 objecves • to...

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